Transistor with contacted deep well region

ABSTRACT

Various methods and devices that involve body contacted transistors are disclosed. An exemplary method comprises forming a gate on a planar surface of a semiconductor wafer. The gate covers a channel of a first conductivity type that is opposite to a second conductivity type. The method also comprises implanting a body dose of dopants on a source side of the gate using the gate to mask the body dose of dopants. The body dose of dopants spreads underneath the channel to form a deep well. The body dose of dopants has the first conductivity type. The method also comprises implanting, subsequent to implanting the body dose of dopants, a source dose of dopants on the source side of the gate to form a source. The method also comprises forming a source contact that is in contact with the deep well at the planar surface of the semiconductor wafer.

CROSS-REFERENCE

The present application is a divisional of U.S. patent application Ser.No. 14/730,092 filed Jun. 3, 2015 and entitled “Transistor withContacted Deep Well Region,” which is incorporated in its entiretyherein by reference.

BACKGROUND OF THE INVENTION

Semiconductor power devices are specialized devices that are used asswitches or rectifiers in power electronics circuits. They arecharacterized by their ability to withstand high voltages and largecurrents as well as the high temperatures associated with high power. Assuch, certain problems faced by standard transistors when they areoperated in high voltage conditions are of particular concern in therealm of power transistors. In addition, the conditions under which atransistor breaks due to the application of large voltages or currentsmust be carefully scrutinized when designing a power device.

Device breakdown is a term used to describe various issues faced bydevices when a certain point is reached at which their performancealters in a nonlinear fashion. One example of device breakdown is the“kink effect” which results from an increase in the body potential of afield effect transistor. However, breakdown is not a phenomenon that islimited to field effect transistors as there are known breakdown modesfor nearly all semiconductor devices including bipolar junctiontransistors, diodes, resistors, capacitors, and generally all transistorand rectifier devices.

The kink effect can be explained with reference to FIG. 1 which includesa cross section 100 of a semiconductor device. The field effecttransistor in cross section 100 provides a conductive path betweensource region 101 and drain region 102 in response to a voltage appliedto the device via an isolated gate electrode in gate stack 103. Theconductive path is formed through body region 104. The conductive pathincludes a channel through body region 104. A schematic of this fieldeffect transistor 105 has been drawn over cross section 100 toillustrate its operation relative to the cross section. The conductivitytypes of source and drain regions 101, 102 are the opposite of theconductivity type of body region 104. As a result, cross section 100 canalso represent a parasitic BJT where body region 104 serves as the baseof the BJT. A schematic of this parasitic BJT 106 has been drawn overcross section 100 to illustrate its operation relative to the crosssection. The kink effect occurs when the parasitic BJT is activated by abuild-up of charge in body region 104 caused by charge carriersassociated with field effect transistor 105 as it is operated in a highpower regime. Although device breakdown is sometimes used intentionallyas part of a design, it is more often a point of operation that designsare meant to avoid.

SUMMARY OF INVENTION

In one embodiment, a transistor is provided. The transistor comprises aninsulating layer located on a planar surface of an active layer. Thetransistor also comprises a gate electrode located on the insulatinglayer. A plan view overlap of the gate electrode and the insulatinglayer encompasses a channel region in a body region in the active layer.The transistor also comprises a source region in contact with the bodyregion and located on the planar surface of the active layer. The bodyregion and the source region have opposite conductivity types. Thetransistor also comprises a deep well region in contact with the sourceregion and the body region. The deep well region and the source regionhave opposite conductivity types. The transistor also comprises a sourcecontact that is in contact with the deep well region and the sourceregion. The source contact is in contact with the deep well region atthe planar surface of the active layer.

In another one embodiment, a method is provided. The method comprisesforming a gate for the transistor on a surface of a wafer. The methodalso comprises forming a dielectric overlayer over the gate. The methodalso comprises patterning the dielectric overlayer with an asymmetricpattern over the transistor. The asymmetric pattern exposes a sourceside of the transistor to form an exposed region of the surface of thewafer on the source side of the transistor but does not expose a drainside of the transistor. The method also comprises implanting a body doseof dopants into the exposed region of the surface of the wafer. The bodydose of dopants spreads underneath the gate to form a deep well for thetransistor. The body dose of dopants has a first conductivity type. Themethod also comprises implanting, subsequent to implanting the body doseof dopants, a source dose of dopants on the source side of thetransistor to form a source region for the transistor. The method alsocomprises implanting, subsequent to implanting the source dose ofdopants, a body contact dose of dopants into the exposed region of thesurface of the wafer. The body contact dose of dopants forms a bodycontact region of the deep well that extends from the surface of thewafer to below the source region. The method also comprises conductingonly one of: (i) forming a source contact for the transistor that is incontact with the deep well at the body contact region; and (ii) forminga source contact and gate shield for the transistor that are in contactwith each other wherein the gate shield is in contact with the deep wellat the body contact region.

In another embodiment, a method is provided. The method comprisesforming a gate on a planar surface of a semiconductor wafer. The gatecovers a channel of a first conductivity type that is opposite to asecond conductivity type. The method also comprises implanting a bodydose of dopants on a source side of the gate using the gate to mask thebody dose of dopants. The body dose of dopants spreads underneath thechannel to form a deep well. The body dose of dopants has the firstconductivity type. The method also comprises implanting, subsequent toimplanting the body dose of dopants, a source dose of dopants on thesource side of the gate to form a source. The method also comprisesforming a source contact that is in contact with the deep well at theplanar surface of the semiconductor wafer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross section of a transistor with circuitschematic elements drawn over the cross section to describe one type ofbreakdown mode in transistors.

FIG. 2 illustrates a cross section of a transistor with a deep well bodycontact.

FIG. 3 illustrates a cross section of a transistor with a deep well bodycontact that is biased via the surface of the semiconductor wafer inwhich the transistor is formed.

FIG. 4 illustrates a cross section of a transistor with a deep well bodycontact and a gate shield that is biased via the surface of thesemiconductor wafer in which the transistor is formed.

FIG. 5 illustrates a flow chart of a set of methods for forming atransistor with a deep well body contact.

FIG. 6 illustrates a panned and zoomed view of the cross section in FIG.3 to highlight certain dimensions of the deep well body contact in FIG.3.

FIG. 7 illustrates a flow chart of a set of methods for forming atransistor with varying deep well surface region patterns.

FIG. 8 illustrates a plan view of a transistor with a contiguous channelwidth deep well surface region pattern.

FIG. 9 illustrates a plan view of a transistor with a deep well surfaceregion pattern.

FIG. 10 illustrates a cross section of two adjacent transistors thatshare a body contact.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Reference now will be made in detail to embodiments of the disclosedinvention, one or more examples of which are illustrated in theaccompanying drawings. Each example is provided by way of explanation ofthe present technology, not as a limitation of the present technology.In fact, it will be apparent to those skilled in the art thatmodifications and variations can be made in the present technologywithout departing from the spirit and scope thereof. For instance,features illustrated or described as part of one embodiment may be usedwith another embodiment to yield a still further embodiment. Thus, it isintended that the present subject matter covers all such modificationsand variations within the scope of the appended claims and theirequivalents.

Semiconductor devices disclosed herein have improved performance in highpower conditions. However, the teachings disclosed herein can be used toimprove semiconductor devices generally and are not limited to highpower applications. Certain methods disclosed herein allow for a costeffective and efficient manner in which to build the disclosed devices.In the particular situations in which the semiconductor device is atransistor, the body of the device is efficiently biased to prevent thedevice from entering into particular breakdown conditions such as thosecaused by an increase in the potential of the body of the transistor.Some of these semiconductor devices are therefore useful in situationsin which the devices are formed in a thin layer of active materialbecause it is in those situations in which the body of the semiconductordevice is more susceptible to the introduction of exogenous charge, asthere is less intrinsic charge in the thin active region to counteractits effect.

FIG. 2 illustrates a cross section 200 of a transistor that is designedfor high voltage applications. Cross section 200 illustrates an activelayer 201 of a semiconductor wafer. Active layer 201 is a region of asemiconductor substrate that includes activated silicon which has hadits conductivity type altered from a base line value in order to be moreconductive for charge carriers having a particular conductivity type. Inthe illustrated example, the remainder of the substrate is not shown,but depending upon the characteristics of the semiconductor wafer,active layer 201 could be located on the insulating layer of an SOIwafer or it could be located on the top of a bulk semiconductor wafer.As illustrated, active layer 201 includes a source region 202 that islocated on a planar surface of active layer 201. Source region 202 isisolated from lightly doped drain region 203 and drain region 204 bybody region 205. An insulating layer 206 is located on the planarsurface of active layer 201. A gate electrode 210 is located oninsulating layer 206. Cross section 200 also includes a secondinsulating layer 207 located on the planar surface of active layer 201.However, insulating layer 206 and 207 may be formed using the same orsimilar materials. In addition, in certain approaches the combination ofinsulating layer 206 and insulating layer 207 can be conceptualized as asingle insulating layer in the finished device regardless of when andhow they are formed.

The general operation of the transistor in cross section 200 can also beexplained with reference to FIG. 2. A conductive path is formed betweensource contact 208 and drain contact 209 in response to the applicationof a voltage to gate electrode 210. Gate electrode 210 is located oninsulating layer 206. The conductive path between source contact 208 anddrain contact 209 includes a channel that is selectively formed in bodyregion 205 under the influence of the aforementioned voltage applied togate electrode 210. While the channel is formed, the transistor is saidto be on. While the channel is not formed, and there is no conductivepath between source contact 208 and drain contact 209, the transistor issaid to be off. There is no conductive path in this situation becausesource region 202 and drain regions 203, 204 have an oppositeconductivity type to body region 205 such that diode junctions areformed at their interfaces. A plan view overlap of gate electrode 210and insulating layer 206 encompasses the channel region in body region205 in active layer 201 when the transistor is on.

The transistor in cross section 200 differs from the transistor in crosssection 100 as it exhibits certain characteristics that make it moresuitable for handling large voltages. For example, cross section 200includes lightly doped drain region 203 which is in contact with bodyregion 205 and is located on the planar surface of active layer 201.Body region 205 and lightly doped drain region 203 are materials ofopposite conductivity types. Lightly doped drain region 203 is lessheavily doped than source region 202. Lightly doped drain region 203 isincluded to enhance the ability of the transistor to withstand largevoltages across source contact 208 and drain contact 209 when thetransistor is off. Cross section 200 also includes a deep well region211 that is in contact with source region 202 and body region 205. Deepwell region 211 extends laterally underneath source region 202 and theportion of body region 205 in which a channel forms. Deep well region211 also enhances the ability of the transistor to withstand largevoltages, but plays a more critical role during the on state of thetransistor. Deep well region 211 serves to remove unwanted chargecarriers from body region 205 to prevent the parasitic bipolar of crosssection 200 from activating.

FIG. 3 illustrates cross section 300 of a semiconductor structure thatis similar to that shown with reference to FIG. 2 with the exceptionthat source contact 208, which was formed in a trench into the planarsurface of active layer 201, has been replaced with source contact 301which does not extend into a trench in active layer 201. Also, deep wellregion 211 has been replaced with deep well region 302 which extends upto the planar surface of active layer 201 and is in contact with sourcecontact 301 at that location. As shown, source contact 301 is in contactwith deep well region 302 at the planar surface of the active layer 201.Since deep well region 302 still extends into body region 205 underneaththe channel of the transistor in cross section 300, deep well region 302is still able to prevent unwanted breakdown effects in the transistor.In addition, in certain approaches, deep well region 302 will be aregion of heavy doping (such as a P+ or N+ region) such that a lowimpedance conductive path is created all the way from source contact 301into a point that is located deep within body region 205.

The transistor of cross section 300, as compared to the transistor ofcross section 200, has an advantage in that etching a trench into activelayer 201 in order to adequately contact body region 205 can introduceadditional costly processing steps and can also degrade deviceperformance. Variability of the trench etch process may lead tovariation in the final device performance which will cause a necessarydowngrading of the minimum performance specifications for all of thedevices produced by a manufacturing process that uses that trench etchprocess. Additionally, stress caused by the trench morphology may causeexfoliation of the semiconductor material in source region 202 duringhigh temperature processing. As a result, the trench may causeadditional deleterious degradation in device performance andmanufacturing line yields. Fabricating the transistor of cross section300 eliminates a trench etching step and provides a higher dose implantfor the deep well contact. As a result, a solid and reliable ohmiccontact can be made to body region 205 without the problems that atrench etch into active semiconductor material can cause.

FIG. 4 illustrates cross section 400 of a semiconductor structure thatis similar to that shown with reference to FIGS. 2 and 3. However, thetransistor in FIG. 4 includes a different source contact structure.Cross section 400 includes a source contact 401 and a gate shield 402.As illustrated, gate shield region 402 is separated from gate electrode210 by a sub-region of dielectric region 206, but is in ohmic contactwith source contact 401. Gate shield 402 is another feature that makesthe transistor in cross section 400 more amenable to high powerapplications. By biasing the gate shield to a given voltage, high powersignals on drain contact 209 are shielded from having an appreciableeffect on the gate. Although gate shield 402 is illustrated as beingohmically coupled to source contact 401, gate shield 402 can also beindependently biased. As drawn, gate shield 402 is distinguished fromsource contact 401 using a horizontal black line as a reminder that thegate shield and upper portion of source contact 401 can be formed in twodifferent steps and can comprise two different kinds of material.However, the horizontal line separating gate shield 402 and 401 isinconsequential to the operation of the device in most situationsbecause the two regions are effectively one contiguous region of highlyconductive material with an uninterrupted ohmic contact from abovedielectric material 207 all the way to the surface of active layer 201.As such, the combination of region 401 and the portion of region 402that underlies region 401 can be conceptualized as a single sourcecontact.

FIG. 5 illustrates a flow chart of a set of methods 500 for forming asemiconductor device with a deep well body contact. Specific methodsbegins with step 501 in which a gate is formed for a transistor. Thegate is formed on the surface of a wafer such as the wafer in whichactive layer 201 is formed in the examples above. As above, the wafercan be a bulk semiconductor wafer or an SOI wafer. The gate can includegate electrode 210 in the examples above. Certain methods continue withstep 502 in which a dielectric overlayer is formed over the gate. Thedielectric overlayer can include the darkened portions of insulatorlayer 206 from the examples above. The overlayer can alternatively takeon any kind of variant shape so long as it provides adequate electricalisolation for the gate electrode from adjoining circuitry.

Formation of the dielectric overlayer can include the application of apatterning process either before or after the formation of the materialused for that layer. For example, certain methods continue with step 503in which a mask is formed on the insulating overlayer. In a specificexample, a photoresist is deposited on the insulating overlayer in orderto create this pattern. Certain methods then continue with step 504 inwhich the insulating overlayer is patterned. The pattern can besymmetrical, or it can be asymmetrical to expose a source side of thetransistor while keeping a drain side of the transistor covered. Theasymmetrical approach allows for the ultimate creation of low dopeddrain regions such as lightly doped drain 203, and also allows foradditional improvements. The dielectric overlayer can be etched usingthe photoresist as a mask to form an exposed region on the surface ofthe wafer on the source side of the transistor. The etch used in thisprocess can be selective to the semiconductor material of the substratesuch that it stops on the planar surface of the substrate and does notform a trench. The etch process can also involve the use of multipleetch materials to prevent damage to the surface of the semiconductorwafer.

Once an exposed region of the surface of the wafer has been created,certain methods continue with step 505 in which a body dose of dopantsis implanted into the exposed region of the surface of the wafer. Thebody dose of dopants then spreads underneath the gate to form a deepwell for the transistor. The body dose of dopants has a firstconductivity type. The deep well spreads down underneath the gate acertain distance from the planar surface of the wafer as controlled bythe pattern.

After the body dose of dopants is implanted, certain methods continuewith step 506 in which a source dose of dopants is implanted on thesource side of the transistor to form a source region for thetransistor. The source dose of dopants can be implanted using the samepattern used for the body dose of dopants or a different pattern.Different patterns for the source dose of dopants are described belowwith reference to FIGS. 8 and 9. In general, the source dose of dopantsis, at least partially, conducted into a region that received the bodydose of dopants. As a result, the source dose of dopants has to bestrong enough to reverse the impact of the body dose of dopants at thatlocation.

After the source dose of dopants is implanted, specific methods continuewith step 507 in which a body contact dose of dopants is implanted onthe source side of the transistor. The body contact dose of dopantsforms a body contact region of the deep well that extends from thesurface of the wafer to below the source region. Using FIG. 4 as anexample for explaining the result of these steps, step 507 will form thesurface portion 403 of deep well region 302 in FIG. 4; while step 506will form source region 202 across the upper surface of active layer 201through to the left side of cross section 400. As such, the upperportion 403 of deep well region 302 was a part of source region 202after step 505, but then became part of deep well region 302 after theexecution of step 507.

Method 500 concludes with the execution of only one of steps 508 and509. Step 508 is utilized to produce a cross section such as the one inFIG. 3 while step 509 is utilized to produce a cross section such as theone in FIG. 4. In step 508, a source contact for the transistor isformed in contact with the deep well at the body contact region. In step509, a source contact and gate shield for the transistor are formed. Thegate shield and source contact are in contact with each other. The gateshield is in contact with the deep well region at the body contactregion. Step 509 is drawn as the final step in the process, but portionsof step 509 directed to the formation of the gate shield could have beenconducted at an earlier step in the process. For example, the portion ofthe gate shield that overlies that gate could be formed just after step502 such that the gate shield is isolated from the gate electrode.However, if step 502 involves the deposition of another isolation layerand the gate electrode has already been isolated at that time, the keyportions of step 509 could be conducted prior to step 502.

Method 500 can also include the steps required to form a lightly dopeddrain region for the transistor such as lightly doped drain region 203in FIG. 3. The required steps can be conducted after step 501, butotherwise can be interspersed throughout the method at any convenientpoint. The doping of the lightly doped drain region is generallyindependent of the doping that is conducted in steps 505-07 becausedevices such as the one shown in cross section 300 are asymmetric andthe drain and source sides of the device are formed, at least in part,using separate steps. The lightly doped drain implant will generally beless heavy than the implant used in step 506 because the lightly dopeddrain region will generally have a lower dopant concentration than thesource region. Since the step can be conducted at various points inmethod 500, step 510 of implanting a lightly doped drain dose of dopantsinto the surface of the wafer on the drain side of the transistor isdrawn using a dotted line to indicate it can be interspersed within themain branch of method 500 at various locations. Step 510 is conductedusing a dopant that causes the semiconductor material in which it isimplanted to have an opposite conductivity type to that of the deep wellregion.

FIG. 6 displays cross section 600 which is a zoomed and panned versionof cross section 300. The relative doping concentrations of the variousimplants in the processes illustrated by method 500 and the commensuratebenefits of specific instances of such processes can be described withreference to the dimensions in cross section 600. Dimension 601illustrates the location of the furthest expanse of the surface region602 of deep well region 302 relative to gate electrode 210. Asillustrated, the surface region 602 is located entirely within 1 micronof gate electrode 210. Dimension 603 illustrates the width of surfaceregion 602 measured away from gate electrode 210. Note that in certainembodiments, which will be described below, the width of surface region602 is larger at some points because it extends all the way through tobody region 205 along the surface of the active layer. However, suchembodiments are still in keeping with dimension 603 as the width of thesurface region because dimension 603 is meant to indicate the minimumnon-zero width of the surface region as measured at the midpoint of thewidth of the channel (i.e., it ignores edge dimensions and any “fingers”of surface region 602). As illustrated, surface region 602 is about 0.4microns wide. In other approaches, surface region 602 is less than 1micron wide as measured in accordance with dimension 603. Finally,dimension 604 illustrates the distance between the gate electrode andthe surface region. As with dimension 602, dimension 604 ignores the“fingers” of the embodiments discussed below, and any irregularitiesaround the ends of the channel. As such, dimension 604 is meant toillustrate a maximum distance from the edge of surface region 602 to thegate electrode as measured away from the gate electrode. Note that gatedielectric 206 is generally much thinner than it appears in FIG. 6 andis negligible for these measurements such that each can be considered asif measured along the planar surface of the wafer. Finally, dimensions601, 603, and 604 are measured normal to the width of the transistor.

In specific methods that are represented by FIG. 5, the body contactdose of dopants is conducted into the surface of the semiconductor waferat a point that has already received the source dose of dopants. As aresult, the body contact dose of dopants has to be strong enough tocounteract the source dose of dopants. In order to properly bias bodyregion 205, this concentration needs to be high. In order to consume theleast amount of surface area on the wafer, and therefore be the mostcost effective, the body contact should be formed in a vertical column.However, regardless of how well an implant is directed, when a dopantspecies is introduced to the surface of a wafer it expands bothvertically and laterally. Therefore, depending upon the thickness ofsource region 202, the implant used to form contact region 602 may haveto spread for some distance laterally in order for it to form a contactfrom the surface of the wafer and down through source region 202 to adepth sufficient to provide a reliable pathway to bias body region 205.

Due to the countervailing desires of keeping the surface area of thecontact limited while at the same time providing a reliable pathway tobias the body of the semiconductor device as well as a suitable sourceregion for the transistor, deep well region 302 provides a significantadvantage when used in combination with a body contact dose of dopants.Since deep well region 302 already provides a reliable conductivepathway from under the source region to under the channel region, thebody contact dose only needs to be high enough to form a hole through tothe bottom of source region 202. For example, the body contact dose ofdopants can be less than 10 times as heavy as the source dose ofdopants. At that point, the preexisting deep well region 302 provides asufficiently high dose of dopants to the overall body contact toefficiently bias body region 205. As a result of this process, dimension603 can be limited to less than 1 micron. Indeed, the inventors havedetermined that dimension 603 can be 0.4 microns, and dimension 601 canbe less than 1 micron. Furthermore, the energy used for the body contactdose of dopants, when conducted into a silicon substrate using commondopant species, can be limited to less than 20 keV while still producingan adequate body contact to the device. However, the implant can be BF₂or As at 30 keV and dimension 603 can still be less than 1 micron.Dimension 604 generally needs to be at least 0.2 microns because sourcecontact 301 needs to contact source region 202 as well as the surfaceregion of the deep well.

FIG. 7 is a flowchart illustrating an additional set of methods 700 thatare variations of the methods discussed with reference to FIG. 5. Ingeneral, the body dose, source dose, and body contact dose can each usedifferent masks and, as will be described below, the body contact dosecan optionally be omitted. In particular, if step 701 is utilized topattern the source dose properly, step 507 can be skipped in itsentirety.

The methods of flow chart 700 involve a patterned implant of the bodycontact dose or the source dose in order to pattern the surface regionof the deep well relative to the source region. In one subset of methodsthat utilize step 701, a patterning mask is created in order to patternthe source implant in step 506. In specific approaches, this stepinvolves the deposition of a photoresist material on the exposed regionof the surface of the semiconductor wafer. The photoresist material isthen patterned to form a finger pattern having a plurality of stripsthat extend away from the channel. Then, the implanting of the sourcecontact dose in step 506 utilizes the finger pattern to form a pluralityof fingers along a width of the transistor channel. An example of thisfinger pattern is described below with reference to FIG. 9. This set ofapproaches can then continue with step 702 and proceed with step 507, orstep 507 can be skipped. This is because the patterned implant of thesource region in step 506 will have already created a surface region ofthe deep well by leaving the original body dose in place underneath themask. However, in certain approaches, another pattern, such as thenegative of the pattern used in step 701, will be applied to allow for asurface region of the deep well with a different characteristic thanwhat resulted from step 505.

In another subset of methods that utilize step 702, a patterning mask iscreated in order to pattern the body contact implant. In specificapproaches, this step involves the deposition of a photoresist materialon the exposed region of the surface of the semiconductor wafer. Thephotoresist material is then patterned to form a contiguous channelwidth pattern. This subset of methods continues with step 507 in whichthe second patterned photoresist is used to form a channel width patternthat extends contiguously along the width of the channel. The implantingof the body contact dose of dopants utilizes the contiguous channelwidth pattern such that the source region isolates a channel of thetransistor from the body contact region.

FIGS. 8 and 9 illustrate plan views of semiconductor devices that havebeen fabricated in accordance with methods from flow chart 700. The planviews can correspond to cross sections 300 and 400 prior to theformation of the source contacts. In each diagram, the gate stack istransparent in order for the lower layers to be seen. However, dottedlines 801, 901 illustrates where the gate stack is located.

FIG. 8 illustrates a plan view 800 of the results of the sub-set ofmethods associated with conducting step 702 in combination with achannel width pattern. Surface region 802 of the deep well region is theportion of the deep well region that is located on the planar surface ofthe wafer. As drawn, source region 202 isolates the portion of bodyregion 205 in which a channel of the transistor is formed from thesurface region. This approach does not require a sophisticated patternand can provide adequate bias to body region 205 through the deep wellregion.

FIG. 9 illustrates plan view 900 of the results of the sub-set ofmethods associated with conducting step 701 in combination with a fingerpattern. Surface region 902 of the deep well region is again the portionof the deep well region that is located on the planar surface of thewafer. Unlike in plan view 800, the source region 202 of plan view 900does not isolate the portion of body region 205 in which a channel isformed from the surface region. Instead, fingers of body contactmaterial 903 lead directly to the portion of the body region in whichthe channel is formed when the transistor is in the on state. In planview 900, a plurality of fingers 902 are located along a width of thetransistor. The deep well region comprises a buried region and a surfaceregion 902. As illustrated, each finger 903 in the plurality of fingersis a strip of the surface region 902 interspersed along the width of thetransistor between a first and a second strip of the source region 202.Therefore, when the source contact is deposited on the planar surface ofthe wafer, the source contact will be in contact with the surface region902 and can bias the body directly through the fingers 903.

The approaches described with reference to FIG. 9 provide a more directpath for biasing the body region of the transistor which makes thedevice even less susceptible to high power breakdown. However, thenumber of fingers should be limited to a reasonable number such thatthey do not limit the overall width of the transistor and thereby reducethe conductivity of the transistor. In most approaches, the total widthof all of the fingers measured along the length of the trench should belimited to less than 5% of the overall width of the transistor. It isalso beneficial to evenly distribute the fingers across the entire widthof the transistor.

FIG. 10 illustrates a cross section 1000 which is a panned and zoomedversion of cross section 300. In a situation in which the transistor incross section 300 shares a source contact 1001 with an adjacenttransistor 1002. Adjacent device 1002 is a mirror image of thetransistor in cross section 300. Not all implementations of crosssection 300 will include adjacent transistor 1002. However, thoseimplementations that do include an adjacent transistor 1002 benefit fromthe low area requirements of the surface region of the body contact onthe planar surface of the wafer. As shown, the size of the surfaceregion of the body contact is a limiting factor to the density of theoverall device. Therefore, the approaches discussed with reference toFIGS. 3 and 6 are particularly beneficial to tightly packed devicesbecause surface region 602 can be limited to less than 0.4 microns andthe combined minimum spacing set by dimension 602 and an overlapping setof dimensions 601 mean that spacing between adjacent devices can be lessthan 1.6 microns. In the illustrated approach, all of deep well region302 is biased by shared contact 1001.

The characteristics of the source contact 1001, source region 1003, andthe relative characteristics of source region 1003 and source contact1001 can exhibit the same characteristics of source region 202 and bodycontact 302 mentioned above with reference to the other figures. Forexample, source region 1003 can isolate surface region 602 from the bodyof adjacent transistor 1002. As another example, source region 1003 caninclude interspersed fingers of surface region material that can providea direct contact to the body of adjacent transistor 1002.

While the specification has been described in detail with respect tospecific embodiments of the invention, it will be appreciated that thoseskilled in the art, upon attaining an understanding of the foregoing,may readily conceive of alterations to, variations of, and equivalentsto these embodiments. These and other modifications and variations tothe present invention may be practiced by those skilled in the art,without departing from the spirit and scope of the present invention,which is more particularly set forth in the appended claims.

What is claimed is:
 1. A transistor comprising: an insulating layerlocated on a planar surface of an active layer; a gate electrode locatedon the insulating layer, wherein a plan view overlap of the gateelectrode and the insulating layer encompasses a channel region in abody region in the active layer; a source region in contact with thebody region and located on the planar surface of the active layer,wherein the body region and the source region have opposite conductivitytypes; a deep well region in contact with the source region and the bodyregion, wherein the deep well region and the source region have oppositeconductivity types; a source contact that is in physical contact withthe deep well region and the source region; wherein the source contactis in physical contact with the deep well region at the planar surfaceof the active layer and a gate shield region separated from the gateelectrode by a first dielectric region; wherein: the source contactprovides an ohmic contact to the gate shield region; in a cross sectionview of the transistor, a first region of the gate shield region extendsover an entirety of the gate electrode, a bottom surface of the firstregion of the gate shield region being above a top surface of the gateelectrode; and in the cross section view of the transistor, a secondregion of the gate shield region extends over a portion of the planarsurface of the active layer, a bottom surface of the second region ofthe gate shield region being below and offset from a bottom surface ofthe gate electrode and being separated from the planar surface of theactive layer by a second dielectric region.
 2. The transistor of claim1, further comprising: a plurality of fingers located along a width ofthe body region; wherein the deep well region comprises a buried regionand a surface region; wherein the source contact is in physical contactwith the surface region; and wherein each finger in the plurality offingers is a strip of the surface region interspersed along the width ofthe body region between a first and a second strip of the source region.3. The transistor of claim 1, wherein: the deep well region comprises aburied region and a surface region; the source contact is in physicalcontact with the surface region; and the source region isolates the bodyregion from the surface region.
 4. The transistor of claim 3, wherein:the surface region is less than 1 micron wide as measured away from thebody region; and the surface region is at least 0.2 microns away fromthe gate electrode.
 5. The transistor of claim 4, wherein: the sourcecontact is shared by an adjacent transistor having a second sourceregion; and the second source region isolates the body region from thesurface region.
 6. The transistor of claim 1, further comprising: alightly doped drain region in contact with the body region, and locatedon the planar surface of the active layer; wherein the body region andthe lightly doped drain region are materials of opposite conductivitytypes; and wherein a dopant concentration of the lightly doped drainregion is less than a dopant concentration of the source region.
 7. Thetransistor of claim 6, wherein in a plan view of the transistor: thegate electrode is located between the source region and the lightlydoped drain region; and a dielectric overlayer covers the gate electrodeand the lightly doped drain region and does not cover the source region.8. A method of forming a transistor comprising: forming a gate on aplanar surface of a semiconductor wafer, wherein the gate covers achannel of a first conductivity type that is opposite to a secondconductivity type, an insulating layer underlying the gate; implanting abody dose of dopants on a source side of the gate using the gate to maskthe body dose of dopants, wherein the body dose of dopants spreadsunderneath the channel to form a deep well, and wherein the body dose ofdopants has the first conductivity type; implanting, subsequent toimplanting the body dose of dopants, a source dose of dopants on thesource side of the gate to form a source; and forming a source contactthat is in physical contact with the deep well at the planar surface ofthe semiconductor wafer; wherein: forming the source contactsimultaneously forms a gate shield for the transistor, the gate shieldbeing separated from the gate by a first dielectric region; the sourcecontact provides an ohmic contact to the gate shield; in a cross sectionview of the transistor, a first region of the gate shield extends overan entirety of the gate, a bottom surface of the first region of thegate shield being above a top surface of the gate; and in the crosssection view of the transistor, a second region of the gate shieldextends over a portion of the planar surface of the semiconductor wafer,a bottom surface of the second region of the gate shield being below andoffset from a bottom surface of the gate and being separated from thesemiconductor wafer by a second dielectric region.
 9. The method ofclaim 8, further comprising: implanting, subsequent to implanting thesource dose of dopants, a body contact dose of dopants to form a bodycontact region of the deep well that extends from the planar surface ofthe semiconductor wafer to below the source; wherein the source contactis in physical contact with the deep well at the planar surface of thesemiconductor wafer via the body contact region.
 10. The method of claim8, further comprising: patterning a mask, prior to the implanting of thesource dose of dopants, to create a pattern comprising a surface regionof the deep well; wherein the source contact is in physical contact withthe deep well at the planar surface of the semiconductor wafer via thesurface region.
 11. The method of claim 10, wherein: the patternisolates the surface region from the channel.
 12. The method of claim10, wherein: the pattern comprises a finger pattern having a pluralityof strips that extend away from the channel along a width of thechannel.
 13. The method of claim 8, further comprising: implanting alightly doped drain dose of dopants into the planar surface of thesemiconductor wafer on a drain side of the transistor to form a lightlydoped drain region; wherein the lightly doped drain region has thesecond conductivity type; and wherein a dopant concentration of thelightly doped drain region is less than a dopant concentration of thesource.
 14. The method of claim 9, wherein: the body contact dose ofdopants is heavier than the source dose of dopants; and the body contactdose of dopants is less than ten times as heavy as the source dose ofdopants.
 15. The method of claim 9, wherein: an exposed region of theplanar surface of the semiconductor wafer receives both the body dose ofdopants and the body contact dose of dopants; and the exposed region ofthe planar surface of the semiconductor wafer has a width of less than 1micron measured away from the gate.
 16. The transistor of claim 1,wherein: a thickness of the second dielectric region is thinner than athickness of a region of the insulating layer that underlies the gateelectrode.
 17. The method of claim 8, wherein: a thickness of the seconddielectric region is thinner than a thickness of a region of theinsulating layer that underlies the gate.